High-strength steel pipe excellent in formability and burst resistance

ABSTRACT

A high-strength steel pipe excellent in formability and burst resistance wherein, when a high-strength steel pipe exceeding 850 N/mm 2  in tensile strength is produced by a UOE method, a ratio (R/r) of an average radius of curvature in a range of 120 mm in a circumferential direction including a weld of the steel pipe before pipe expansion in a pipe expansion process (R) to the radius of the steel pipe after pipe expansion (r) is 0.65 to 2.0, and preferably the ratio (R/r) is 0.90 to 2.0. Preferably, Vickers hardness of the base metal Hv, the minimum Vickers hardness at the HAZ Hz, pipe wall thickness t, and a peaking amount at the weld of the steel pipe before pipe expansion in a pipe expansion process δ satisfy the following expression, 
     
       
         (1+0.005t|δ|) Hz &lt;0.03584 Hv   2 −25.34 Hv +4712.

TECHNICAL FIELD

The present invention relates to a high-strength steel pipe havingimproved formability and burst resistance when the steel pipe is formedby a UOE manufacturing method.

BACKGROUND ART

The group of processes for producing a steel pipe by a UOE method isgenerally composed of the processes of: C-forming (pressing) of a steelsheet, U-forming (pressing), O-forming (pressing), seam welding and pipeexpansion, as shown in FIG. 1. In the C-forming process, beveling isapplied to both the edges of a steel sheet and then bending is appliedto the vicinities of the edges of the steel sheet. The bending is mostlyapplied with press forming, but it is also possible to form bentportions in the vicinities of the edges of a steel sheet with rollforming as disclosed in Japanese Unexamined Patent Publication No.S61-279313. The steel sheet after being subjected to the C-forming isthen formed into a “U-shape” in a U-forming process, and thereafterformed into a tubular shape in an O-forming process. After that, boththe edges of the steel sheet formed into a tubular shape, whose bevelends are in butting relation with each other, are seam-welded in a seamwelding process. At this stage, a pipe closed in the circumferentialdirection is formed for the first time and, then, the pipe is subjectedto pipe expansion using a pipe expansion device called an expander in apipe expansion process for obtaining a better tubular shape, namely forimproving the roundness of the pipe. As the methods of expanding a pipe,there are two methods; the mechanical pipe expansion method wherein adeformation is imposed forcibly from the interior of a pipe towards theexterior thereof, and the hydraulic pressure pipe expansion methodwherein a hydraulic pressure is imposed in the interior of a pipe. Atpresent, the former method is mostly employed. Note that, though thereis the method of reducing the diameter of a pipe from the exteriorthereof for improving the roundness of the pipe in contrast with theabove pipe expansion methods, this method is differentiated from the UOEmethod.

In the past, in the UOE method for producing a steel pipe, manyinventions have been produced for improving formability such asroundness, the capacity of existing facilities and the formability of apipe having a heavy wall thickness by specifying the forming conditionsin each of the processes of C-forming, U-forming, O-forming and pipeexpansion.

For example, in the forming method of C-pressing, Japanese PatentApplication No. H8-294724 discloses a method of reducing peaking (apositive deviation from the concentric circle at a weld) and making itpossible to form a heavy thickness material or a high-strength materialby prescribing a specific relation to the forming length, the yieldstrength of a sheet material and the thickness thereof in the C-formingprocess, without increasing the capacity of C-pressing and/orO-pressing.

Further, Japanese Unexamined Patent Publication Nos. H9-239447 andH10-211520 disclose that bad shapes can be improved even within thecapability of existing facilities by: controlling the length of thebending region to 3.5 times or more the sheet thickness or controllingthe length of the remaining straight portion to 1.5 times or less thesheet thickness when C-forming is applied; and, by so doing, restrictingpeaking (a protrusion at an abutting portion in this technology to 2 mmor less. Yet further, Japanese Patent No. 1135933 discloses a technologythat enables the shape of a steel pipe to be improved by controlling theratio between the radius of curvature at C-pressing (the radius ofcurvature before O-pressing) and the radius of curvature of the steelpipe within the range from 0.8 to 1.2 and, by so doing, reducingpeaking. As technologies developed by the forming conditions inC-pressing as disclosed above, there have been proposed the technologiesdisclosed in Japanese Unexamined Patent Publications No. S55-14724, No.S59-199117 and No. S60-92015.

In addition, as a technology for improving the formability inO-pressing, there is the technology of reducing peaking by forming aheteromorphic portion at the center of a die caliber in the longitudinaldirection as disclosed in Japanese Patent No. 1258977. Moreover, thereare other technologies of improving O-pressing as proposed in JapaneseUnexamined Patent Publications No. H9-94611 and No. S53-112260.

Further, as a technology of correcting roundness and bending by devisinga pipe expansion process, there is the technology of applying pressingseveral times while the relative positions of a caliber and a materialto be formed are changed as disclosed in Japanese Unexamined PatentPublication No. H03-94936. As other technologies, there are thetechnologies of improving roundness in relation to pipe expansion asproposed in Japanese Unexamined Patent Publication Nos. S57-94434 andS61-147930.

In recent years, the importance of a line pipe has been increasing stillmore as a means of long distance transportation of crude oil and naturalgas. In particular, in order (1) to improve a transportation efficiencyby applying a higher pressure and (2) to improve a field constructionefficiency by reducing the outer diameter and weight of a line pipe, theneeds for a high-strength line pipe exceeding X100 (760 N/mm² or more intensile strength) have been getting stronger. To cope with these needs,in recent years, a technology of applying TMCP even to the production ofa steel sheet exceeding 760 N/mm² in tensile strength, which has beendifficult so far (refer to Japanese Unexamined Patent Publication No.H8-199292) has been developed.

In the meantime, as the strengthening of a line pipe advances, it hasbeen clarified that the softening of a heat affected zone (a HAZ), whichhas scarcely been considered until now as a problem, when a medium- orlow-strength material of about 700 N/mm² in tensile strength has beenwelded with submerged arc welding or the like, advances and the criticalplastic strain, at which ductile cracking starts to occur during theforming of a sheet material, lowers when a high-strength materialexceeding 850 N/mm² in tensile strength is used. Therefore, when a linepipe exceeding 850 N/mm² in tensile strength is formed, problems such ascracking and rupture at a weld in a pipe expansion process after seamwelding and brittle rupture at a seam weld when an obtained steel pipeproduct is subjected to an internal pressure load occur. These problemsdid not appear when a conventional medium- or low-strength steel pipewas produced.

DISCLOSURE OF THE INVENTION

The object of the present invention is, in view of problems in theexisting technologies as stated above, to provide a method of producinga high-strength steel pipe so excellent in formability as not to incurcracking and rupture at a weld in a pipe expansion process when such ahigh-strength steel pipe, for line pipe use, exceeding 850 N/mm² intensile strength is produced and so excellent in burst resistance as notto incur brittle rupture at a seam weld even when the steel pipe productis subjected to an internal pressure load during its service.

The present invention has been accomplished for solving theabove-mentioned problems and the gist of the present invention is asfollows:

(1) A high-strength steel pipe excellent in formability, characterizedin that, when a high-strength steel pipe exceeding 850 N/mm² in tensilestrength is produced by a UOE method, the ratio (R/r) of the averageradius of curvature in the range of 120 mm in the circumferentialdirection including the weld of the steel pipe before pipe expansion ina pipe expansion process (R) to the radius of the steel pipe after pipeexpansion (r) is 0.65 to 2.0.

(2) A high-strength steel pipe excellent in formability and burstresistance, characterized in that, when a high-strength steel pipeexceeding 850 N/mm² in tensile strength is produced by a UOE method, theratio (R/r) of the average radius of curvature in the range of 120 mm inthe circumferential direction including the weld of the steel pipebefore pipe expansion in a pipe expansion process (R) to the radius ofthe steel pipe after pipe expansion (r) is 0.90 to 2.0.

(3) A high-strength steel pipe excellent in formability, characterizedin that, when a high-strength steel pipe exceeding 850 N/mm² in tensilestrength is produced by a UOE method, the absolute value of the strainin the circumferential direction at a point 4 mm distant from each ofthe toes of the weld during pipe expansion is 4% or less.

(4) A high-strength steel pipe excellent in burst resistance,characterized in that, when a high-strength steel pipe exceeding 850N/mm² in tensile strength is produced by a UOE method, the absolutevalue of the strain in the circumferential direction at a point 4 mmdistant from each of the toes of the weld during pipe expansion is 2.5%or less.

(5) A high-strength steel pipe excellent in burst resistance,characterized in that, when a high-strength steel pipe exceeding 850N/mm² in tensile strength is produced by a UOE method, the peakingamount before pipe expansion satisfies the expression (1) and at leastthe height of the shrinkage allowance of the weld metal at the innersurface is 2.0 mm or less,

−1.5 mm≦peaking amount (mm)≦16/pipe wall thickness (mm)  (1).

(6) A high-strength steel pipe excellent in burst resistance accordingto the item (5), characterized in that the change in the peaking amountfrom before pipe expansion to after pipe expansion satisfies theexpression (2),

−1.5 mm≦change in peaking amount (mm)≦1.0 mm  (2).

(7) A high-strength steel pipe excellent in burst resistance,characterized in that, when a high-strength steel pipe 900 N/mm² or morein tensile strength is produced by a UOE method, the Vickers hardness ofthe base metal of the steel pipe Hv, the minimum Vickers hardness at theHAZ Hz, the pipe wall thickness t, and the amount of peaking deviatedfrom the perfect circle in the range of 120 mm in the circumferentialdirection including the weld of the steel pipe before pipe expansion ina pipe expansion process δ satisfy the expression (3),

(1+0.005t|δ|) Hz<0.03584Hv²−25.34Hv+4712  (3).

(8) A high-strength steel pipe excellent in burst resistance accordingto the item (7), characterized in that the peaking amount δ satisfiesthe expression (4),

 |δ|<40/t  (4).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a process for producing asteel pipe by a UOE method.

FIG. 2 is a graph showing the relation between the ratio (R/r) of theradius of curvature at a portion in the vicinity of a weld before pipeexpansion (R) to the radius of a steel pipe after pipe expansion (r) andthe occurrence or not of a rupture at the seam weld during pipeexpansion.

FIG. 3 is an illustration showing a relationship between the radius of asteel pipe after pipe expansion (r) and the radius of curvature at aportion in the vicinity of a weld before pipe expansion (R).

FIG. 4 consists of illustrations showing a relationship between asegment for pipe expansion during pipe expansion and the radius ofcurvature at a portion in the vicinity of a weld before pipe expansion(R), and the status of the occurrence of a strain (polarity) during thepipe expansion.

FIG. 5 is a graph showing the relation between the ratio (R/r) of theradius of curvature at a portion in the vicinity of a weld before pipeexpansion (R) to the radius of a steel pipe after pipe expansion (r) andthe form of a rupture in a hydraulic pressure burst test.

FIG. 6 is an illustration showing the method of measuring a strainduring pipe expansion.

FIG. 7 is an illustration showing a model of a welded joint used for thefinite element method.

FIG. 8 is a graph showing the result obtained by analyzing a strain inthe tensiletest.

FIG. 9 is a schematic illustration showing the height of the shrinkageallowance of a weld metal at an inner surface.

FIG. 10 is a graph showing the influence of a peaking amount, and theheight of a weld metal at an inner surface, on burst resistance.

FIG. 11 is an illustration explaining the definition of a peakingamount.

FIG. 12 is a graph showing the relation between a pipe wall thicknessand a critical peaking amount.

FIG. 13 is a graph showing a peaking amount before pipe expansion andthe difference in the peaking amount between before and after the pipeexpansion.

FIG. 14 is a graph showing the relation between the change in thepeaking amount from before pipe expansion to after pipe expansion andthe form of a burst rupture.

FIG. 15 is a graph showing the classification of the form of a rupturein the tensile test of a joint in relation to the hardness of a basemetal and that of a HAZ.

FIG. 16 is a graph showing the classification of the form of a rupturein a burst test in relation to the hardness of a base metal and that ofa HAZ.

FIG. 17 is a graph showing the classification of the occurrence or notof a rupture during pipe expansion in relation to a wall thickness and apeaking amount.

FIG. 18 is a graph showing the relation between a Vickers hardness and atensile strength.

BEST MODE FOR CARRYING OUT THE INVENTION

Firstly, in order to understand the mechanism of generating cracking andrupture at a seam weld in a pipe expansion process when a high-strengthsteel pipe exceeding 850 N/mm² in tensile strength was formed by a UOEmethod, the present inventors produced steel pipes whose curvatures werevariously changed and investigated the occurrence or not of a rupturefrom a weld when pipe expansion was applied under the condition that thepipe expansion ratio was 1%. As a result, it was found that theoccurrence or not of a rupture from a weld during pipe expansion wasrelated to the radius of curvature in the vicinity of a weld before pipeexpansion and the radius of the steel pipe after pipe expansion. FIG. 2shows the relation between the ratio (R/r) of the average radius ofcurvature in the range of 120 mm in the circumferential direction withthe weld of a steel pipe before pipe expansion (after O-pressing andseam welding) as the center (R) to the radius (average circle-equivalentradius) of the steel pipe after pipe expansion (r) and the occurrence ornot of a rupture (expressed by the mark ⋄ when a rupture does not occurduring pipe expansion and by the mark  when a rupture occurs duringpipe expansion) from the weld during pipe expansion. Here, the radius ofa steel pipe after pipe expansion (r) was varied by properly adjustingthe curvature at C-pressing and the upset amount at O-pressing.

From FIG. 2, it was clarified that a rupture occurred at a weld duringpipe expansion when R/r was less than 0.65 or over 2.0 but a rupture didnot occur when R/r was in the range from 0.65 to 2.0.

Further, it was clarified that the influence of the strain concentrationon a weld when a pipe expansion ratio was increased during pipeexpansion was far less than that of the average radius of curvature inthe vicinity of the weld of a steel pipe before pipe expansion (R), theradius of the steel pipe after pipe expansion (r) or the like, and thatthe cracking property of a weld during pipe expansion was substantiallydominated merely by the curvature ratio (R/r).

FIG. 3 is an illustration showing the relation of the positions betweenthe radius of a steel pipe after pipe expansion (r) and the averageradius of curvature in the range of 120 mm in the circumferentialdirection with the weld before pipe expansion as the center (R), whenR/r is in the range between the lower limit (0.65) and the upper limit(2.0) where a rupture does not occur at a weld during pipe expansion.

From the result obtained by observing the ruptured sections, it wasfound that cracking and rupture were generated with the outer side of aweld as the starting point of rupture during pipe expansion when R/r waslarger than the upper limit 2.0 and with the inner side of a weld as thestarting point of rupture when R/r was smaller than the lower limit0.65; respectively.

Therefore, the present invention determines the ratio (R/r) of theaverage radius of curvature in the range of 120 mm in thecircumferential direction with the weld of a steel pipe before pipeexpansion (after O-pressing and seam welding) as the center (R) to theradius (average circle-equivalent radius) of the steel pipe after pipeexpansion (r) to be 0.65 to 2.0, in order to suppress the cracking andrupture of the weld during pipe expansion when the steel pipe isproduced. By so doing, the cracking and rupture at a seam weld, whichare generated in a pipe expansion process when a high-strength steelpipe exceeding 850 N/mm² in tensile strength is formed by a UOE method,can be suppressed and the formability and the production efficiency of ahigh-strength steel pipe can be improved.

A steel pipe such as a line pipe used in an environment where aninternal pressure is imposed is desired not to incur not only crackingand rupture at the weld but also rupture from the weld in an environmentwhere an internal pressure load is imposed during its service. As acriterion, it is desirable that no rupture from a weld but only arupture of a pipe body (base metal) occurs when a steel pipe issubjected to a burst test.

Further, the present inventors carried out a hydraulic pressure bursttest, using steel pipe products which satisfied the above-mentionedcondition that R/r was in the range from 0.65 to 2.0 and thus had a goodformability, without incurring cracking and rupture at the welds duringpipe expansion.

FIG. 5 shows the relation between the ratio (R/r) of the average radiusof curvature in the range of 120 mm in the circumferential directionwith the weld of a steel pipe before pipe expansion (after O-pressingand seam welding) as the center (R) to the radius (averagecircle-equivalent radius) of the steel pipe after pipe expansion (r) andthe occurrence or not of a burst (expressed by the mark ⋄ when a ruptureoccurs at a pipe body, by the mark  when a ductile rupture occurs at aweld and by the mark ★ when a brittle rupture occurs at a weld) when thesteel pipe products are subjected to a hydraulic pressure burst test.From FIG. 5, it was found that a burst (rupture) from a weld occurredwhen R/r was less than 1 and a burst (rupture) from a pipe body occurredwhen R/r was 1 or more.

FIG. 4 shows the relation of the positions between a segment (the radiusof curvature corresponds to the radius of a steel pipe (r)) for pipeexpansion which is used during pipe expansion and the average radius ofcurvature in the range of 120 mm in the circumferential direction withthe weld before pipe expansion as the center (R), and the status of astrain generated during the pipe expansion. From FIG. 4, it wasunderstood that the tensile strain caused by bending during pipeexpansion concentrated on the inner side of a steel pipe under thecondition that R/r was less than 1 and the tensile strain caused bybending during pipe expansion concentrated on the outer side of a steelpipe under the condition that R/r was 1 or more.

In addition, as a result of a numerical analysis with the finite elementmethod by the present inventors, it was found that, under the conditionthat R/r was less than 1, an excessive plastic strain remained at eachof the toes of the weld at the inner surface of a steel pipe caused by abending load during pipe expansion and the amount of the plastic strainexceeded 25%. Therefore, it is considered that the mechanism of theburst at the weld of a steel pipe is as follows: when a steel pipe issubjected to pipe expansion under the condition that R/r is less than 1during the forming of the steel pipe, an excessive plastic strainremains at the weld of the steel pipe, the amount of tensile straincaused by the internal pressure load of the pipe during the use of thesteel pipe is added to the amount of the residual strain, the totalstrain reaches the critical rupture strain and, as a result, a burst(rupture) occurs from the toes of the weld. On the other hand, when R/ris 1 or more, the residual strain at a weld caused by bending duringpipe expansion acts on the compression side, thus the amount of thestrain remaining at the toes of the weld on the inner side of the weldafter pipe expansion goes down even in a compressive environment or atensile environment, as a result, the amount of the plastic strainbecomes overwhelmingly smaller than that of a steel pipe expanded underthe condition that R/r is less than 1 during the forming of the steelpipe even when the amount of the tensile strain caused by the internalpressure load of the pipe during the use of the steel pipe is added tothe amount of the residual strain, and therefore a burst (rupture) fromthe inner side of the weld during the use of the steel pipe issuppressed. Note that, in this case, though a rupture from the outerside of the weld of a steel pipe is apt to occur, as the state of thestress of the steel pipe when an internal pressure is loaded on thesteel pipe during the use of the steel pipe is such that the strain ofthe outer side is more mitigated than that of the inner side, thestrength of the weld against a rupture improves as a whole.

As explained above, by adjusting the condition of R/r during pipeexpansion when a steel pipe is produced, it is made possible to controlthe amount of a strain (the amount of a residual strain) generated ateach of the toes of a weld at the inner and outer surfaces of a steelpipe during pipe expansion, and by adjusting the polarity of the strain,to reduce the amount of the critical rupture plastic strain generatedcaused by an internal pressure load during the use of the steel pipe,and to suppress a burst at the weld (to attain the burst of the pipebody).

Further, the ruptured sections of the test pieces which had burst fromthe welds in a hydraulic pressure burst test were observed, and it wasfound that the rupture was a ductile rupture in the case of the testpieces having R/r of 0.9 or more to less than 1.0 and the same was abrittle rupture in the case of the test pieces having R/r of less than0.9.

In case of a steel pipe for line pipe use, a brittle rupture inparticular, among the forms of cracking from welds, must be avoided,because a brittle rupture has a high cracking propagation velocity and alow cracking propagation stopping capability and thus is a factor incausing a large breakage of a line pipe. For this reason, the presentinvention determines the ratio (R/r) of the average radius of curvaturein the range of 120 mm in the circumferential direction with the weld ofa steel pipe before pipe expansion (after O-pressing and seam welding)as the center (R) to the radius (average circle-equivalent radius) ofthe steel pipe after pipe expansion (r) to be 0.9 to 2.0, in order tosuppress a brittle rupture at the weld of the steel pipe in theenvironment where the steel pipe is used for a line pipe. Preferably, inorder to perfectly avoid a rupture from the weld of a steel pipe in theenvironment where the steel pipe is used for a line pipe, R/r must bedetermined to be 1.0 to 2.0.

Further, in the present invention, based on the knowledge that the pipeexpansion cracking during pipe expansion and the seam burst during theuse of a steel pipe were originated from the toes of a weld at the innersurface and that an angular distortion influenced burst resistance,strain gages were attached to the points 4 mm apart from the toes of theweld at the inner surface of a steel pipe as shown in FIG. 6 and thestrains in the circumferential direction during pipe expansion weremeasured. The strains were measured until the strains reached themaximum pipe expansion ratio continuously during pipe expansion or untilpipe expansion cracking occurred. When R/r was 1 or less, the strainssimply increased to the tensile side in general and, when R/r was 1 ormore, the strains once took the state of compression and then turned tothe tensile side. Here, the maximum strain amounts and the forms ofruptures were compared in the pipe expansion process. As a result, whena tensile strain exceeded 4%, pipe expansion cracking from a weldoccurred in many test samples. Based on that, the present inventorsinvented a technology that made it possible to prevent pipe expansioncracking by controlling the absolute value of a strain at a point 4 mmapart from each of the toes of a weld to 4% or less.

Some steel pipes among those that succeeded in pipe expansion weresubjected to a hydraulic pressure burst test, and the strains measuredduring pipe expansion and the forms of burst ruptures were compared. Asa result, it was found that a burst occurred from a seam weld frequentlywhen a pipe expansion strain exceeded 2.5%. On the other hand, a burstoccurred from a pipe body without exception when a pipe expansion strainwas 2.5% or less. Therefore, by controlling the absolute value of astrain at a point 4 mm apart from each of the toes of a weld to 2.5% orless, it is made possible to supply a steel pipe which is prevented fromruptureing at a seam weld caused by a burst when an inner pressure isimposed.

The reasons why the position where a strain is controlled is determinedto be a point 4 mm apart from each of the toes of the weld at an innersurface are as follows: the measurement of a strain is not affected byC-pressing, U-pressing or O-pressing in the vicinity of an edge of asheet; the critical equivalent plastic strain amount which can be usedas an indicator of the generation of ductile cracking is not affected byother production processes; the strain amount measured at the positioncan represent the macroscopic strain amount in the vicinity of each ofthe toes of a weld; and a softened band of a HAZ exists at a position 2to 3 mm apart from each of the toes of a weld and the measurement of astrain by attaching a strain gage there is apt to produce an error. Itis also possible to set the position of strain measurement at a positionmore distant from a 4 mm apart position though the accuracy is inferior,and in that case, the strain may be controlled with the inverseproportion to the distance from each of the toes of a weld.

The present inventors ran a numerical analysis simulation with thefinite element method (hereunder referred to as “FEM”) in order toinvestigate the influence of the shape of a weld, the strength of a basemetal, the strength of a weld metal, the strength of a HAZ and the widthof a HAZ on the strength of a weld joint. Table 1 shows the analysisconditions, FIG. 7 the model of the weld joint used for FEM on the scaleof one fourth, and FIG. 8 the result of the calculation.

TABLE 1

FIG. 8 shows that a joint breaks when an equivalent plastic strainreaches a critical limit. When the displacements are identical, that astrain amount is larger means that the strain is concentrated more. Fromthis fact, it is understood that, even if the shapes of bevels areidentical, the higher the height of the reinforcement of a weld metalis, the more the concentration of the strain is, and, even if theheights of weld metals are identical, the larger the bevel angle is, theless the concentration of the strain is. The slight variation in thecritical strain amount in each case is caused by the influence of thedegree of a triaxial stress. It was found that, only in case 2, theequivalent plastic strain reached the critical strain, but, in cases 1and 3, the strain concentrated on the base metal before the equivalentplastic strain at each of the toes of the weld reached the criticalstrain and thus no rupture occurred at the weld.

Then high-strength steel pipes 914 mm in outer diameter and 16 mm inwall thickness were subjected to an inner pressure burst test while theheight of the reinforcement of the deposited metal at the inner surfaceof each of the welds was varied. As a result, the form of a rupture wasnot always the form of the rupture dependent on the height of thereinforcement of a deposited metal. Here, the height of thereinforcement of a weld metal means the height thereof from the innersurface of a pipe as shown in FIG. 9. As a result of observing theruptured sections of the test samples which ruptured from the welds, itwas found that most of the test samples incurred brittle ruptures orductile ruptures which originated from the inner surfaces and the burstsoccurred at a certain stage in the process of raising pressure,according to the graph showing the relation between the inner pressureand the time. This means that the withstanding pressure of a weldedsteel pipe is lower than that intrinsic to a base metal (lower than thewithstanding pressure of a pipe body). The relation between a peakingamount and the height of a weld at an inner surface is shown in FIG. 10based on the presupposition that a rupture is apt to occur because aplastic strain concentrates at each of the toes of a weld at an innersurface during pipe expansion when a positive peaking exists before pipeexpansion as stated above. Here, a peaking amount is defined based onFIG. 11. That is, a peaking amount is defined as the distance between atoe of a weld and the nominal outer surface of a pipe prepared so as tocross the actual outer surface of the pipe at the positions 60 mmdistant from either of the toes of the weld. When a peaking amount ismeasured after tackwelding, the peaking amount may be defined by thedistance between an edge of a bevel and the nominal outer surface. As aresult, it was found that, even though the height of a metal at an innersurface was 2.0 mm or less, the burst pressure was lower than thewithstanding pressure of a pipe body when the peaking amount exceeded1.0 mm.

On the other hand, when test samples 2.0 mm or less in height of a metalat an inner surface and 1.0 mm or less in peaking amount were subjectedto a hydraulic pressure burst test, the increase of the pressure withthe lapse of time was not observed in spite of the boosting of thepressure by a pump, and the test samples broke either directly or aftera slight reduction in the pressure. This means that the stress of thebase metal has reached the tensile strength, the withstanding pressureis sufficient for a practical use, and the weld has a withstandingpressure identical to that of the pipe body.

Based on this, the present inventors discovered that a burst strengthidentical to that of a pipe body could be obtained by controlling theheight of a weld metal at an inner surface to 2.0 mm or less and apeaking amount before pipe expansion to 1.0 mm or less. That is, apeaking amount must satisfy the expression; −1.5 mm≦peaking amount(mm)≦1.0 mm.

When a peaking amount was lower than −1.5 mm, a burst occurred with apressure lower than the withstanding pressure of a pipe body even if theheight of a weld metal at an inner surface was within the rangespecified in the present invention. As a result of investigating theruptured section, it was clarified that the rupture originated from theouter surface of the weld metal. Therefore, the effects of the presentinvention are not demonstrated when a peaking amount is lower than −1.5mm. Generally speaking, the bevel stability during O-pressingdeteriorates as a peaking amount goes down towards a negative value and,if a peaking amount is lower than −2.0 mm, buckling is apt to occur andstable forming is hardly attained in a mass production.

Next, the present inventors investigated whether or not the rangesspecified in the present invention could be applicable to a pipe havinganother wall thickness and outer diameter. FIG. 12 shows the result ofthe burst test in the case where the height of the reinforcement at aninner surface is 2.0 mm or less. The thicker the wall thickness is, thelower the critical peaking amount is, and a positive peaking amount inwhich a withstanding pressure identical to that of a pipe body isobtained is determined by the value of 16/pipe wall thickness (mm). Thatis, a peaking amount must satisfy the expression; −1.5 mm≦peaking amount(mm)≦16/pipe wall thickness (mm). It is desirable to control a peakingamount in the range from 0 to 16/pipe wall thickness (mm) in order tosecure a stable production also in a mass production.

The rupture at the seam weld of a test sample having had a positivepeaking value before pipe expansion originated from the inner surface,and the rupture of a test sample having had a negative peaking valueoriginated from the outer surface. The present inventors thought thatthe resistance of a weld against a rupture by burst was caused by theconcentration of a plastic strain on the toes of the weld and the HAZand further the absolute value thereof mainly depended on the amount ofchange in peaking amount from before pipe expansion to after pipeexpansion. Based on that, the peaking amounts before and after pipeexpansion were measured and the relation between a peaking amount beforepipe expansion and the amount of change in the peaking amount frombefore pipe expansion to after pipe expansion was obtained as shown inFIG. 13. From the figure, it was found that, though the peaking beforepipe expansion came close to the nominal surface of a pipe, whichconstituted the target curvature, by applying pipe expansion, the datalargely distributed in the direction of too large correction (to theside where the amount of change in the peaking amount was larger thanthe line showing the nominal surface of a pipe in FIG. 13).

Among the test samples, the test samples each having the reinforcement2.0 mm or less in height at the inner surface of the weld were extractedand were subjected to a burst test. The results are shown in FIG. 14,taking the test samples 914 mm in diameter and 16 mm in thickness asexamples. The test pieces were classified into the following threecategories in relation to the withstanding pressures and the forms ofruptures: the ones which had the withstanding pressures lower than thoseof the pipe bodies and incurred seam bursts; the ones which had thewithstanding pressures identical to those of the pipe bodies andincurred seam bursts; and the ones which had the withstanding pressuresidentical to those of the pipe bodies and incurred bursts from the pipebodies. As a result, it was found that, even though the peaking amountsbefore pipe expansion were in the range from −1.5 to 1.0 mm, the testsamples having the amounts of change in peaking amount exceeding 1 mmduring pipe expansion incurred seam bursts, but the test samples havingthe amounts of change in peaking amount in the range from 1 mm or lessto −1.5 mm or more incurred bursts from the pipe bodies.

The reason why the burst resistance improves as the amount of change inpeaking amount reduces is that the amount of change in peaking amountmost affects the concentration of a strain on a weld. The reason why theallowance of a peaking amount is larger on the negative side is that thecompressive strain caused by angular distortion is offset against thetensile strain in the circumferential direction and, as a result, theamount of equivalent plastic strain reduces.

Besides a peaking value, the pipe expansion ratio is given as anindicator of formability showing the concentration of a strain on aweld. However, in order to secure the roundness of a whole pipe, thepipe expansion ratio cannot be lowered, and, in order to control theroundness, as defined by the American Petroleum Institute, to within ±1%of a nominal diameter, it is necessary to secure the pipe expansionratio of 0.7% or more. Usually a pipe expansion ratio of 0.8 to 1.2% isemployed. The angular distortion for correcting a peaking makes theconcentration of a strain on each of the toes of a weld or a HAZ farlarger than the increment of a strain caused by the increment of thepipe expansion ratio, and therefore it may be said that the strength ata weld is substantially dominated by a peaking.

Then the present inventors flattened the test pieces containing thewelds of high-strength steel pipes exceeding 900 N/mm² in tensilestrength and subjected them to a tensile test in the directionperpendicular to the weld lines. As a result, in case of medium-strengthmaterials (X-65, X-80) and high-strength materials (800 N/mm² class intensile strength), the test pieces ruptured from the base metals and, incase of a steel pipe exceeding 900 N/mm² in tensile strength, the testpieces mostly ruptured from the welds. Further, as a result of observingthe ruptured sections in detail, it was found that there were two formsof ruptured sections, namely ductile ruptures and brittle ruptures.Here, the conditions of the forming, the strength of he base metal, thestrength of the HAZ, the shape of the weld, the conditions of thewelding and the like of each test piece were analyzed in detail. As aresult, it was found that a brittle ruptured section and a ductileruptured section could be differentiated by maintaining the relationbetween the Vickers hardness of a base metal and that of a HAZ within aspecific range. Here, the Vickers hardness of a base metal isrepresented by the average hardness of the pipe body material on theside where a rupture at a weld originates, and the Vickers hardness of aHAZ is meant as the minimum hardness at the HAZ on the side where arupture at the weld of a pipe originates and the position showing theminimum hardness exists generally within 3 mm from each of the toes of aweld. The starting point of a rupture at a weld has a close relationwith a peaking amount before pipe expansion, and, in case of a positivepeaking amount, a rupture originates at the inner surface and, in caseof a negative peaking amount, it originates at the outer surface. Thatis, as long as the hardness of a base metal, the hardness of a HAZ, apeaking amount and a pipe wall thickness satisfy the followingexpression (3), the ruptured section shows a ductilely ruptured section:

(1+0.005t|δ|)Hz<0.03584Hv ²−25.34Hv+4712  (3),

where,

Hv: Vickers hardness of base metal,

Hz: Vickers hardness of HAZ,

δ: peaking amount before pipe expansion, mm,

t: pipe wall thickness, mm.

The present inventors perceived that the position of a rupture varieddepending on allowance in peaking amount and that the magnitude of thepeaking amount affected the form of a rupture, and deduced theexpression (3). When a peaking amount is positive, a strain concentratespredominantly on a HAZ at an inner surface during pipe expansion, but,when a peaking amount is negative, a strain concentrates predominantlyon a HAZ at an outer surface. When a steel pipe thus having incurred aplastic strain is flattened and then subjected to a tensile test, theinfluence of the plastic strain having remained during pipe expansion ispredominant and the starting point of cracking is generated depending onallowance in peaking amount. A large peaking amount means that a plasticstrain amount incurred during pipe expansion is large, and it isestimated that, when such a test piece is subjected to a tensile test,the base metal reaches the critical strain amount without generatingpredominant elongation, ductile cracking occurs, and, right after that,a brittle rupture occurs. The present inventors analyzed an equivalentplastic strain amount generated at a HAZ during pipe expansion with FEMand confirmed that the equivalent plastic strain amount exceeded 25% andthere was no allowance to the critical strain amount.

Then, steel pipes, each of which being cut out from the place adjacentto the place where a test piece for tensile test of a weld joint wastaken, were subjected to an inner pressure burst test. FIG. 16 shows thefracture portion obtained by subjecting the steel pipes 914 mm in outerdiameter and 16 mm in wall thickness tothe burst test, together with theresults of the tensile test of weld joints shown in FIG. 15. The formsof ruptures in the burst test are classified into two categories; arupture from a weld and a rupture at a pipe body. The test pieces whichshowed the ruptures at the pipe bodies corresponded with the steel pipeswhich showed ductilely ruptured sections in the tensile test of the weldjoints and the test pieces which showed the ruptures at the seam weldscorresponded with the steel pipes which showed brittlely rupturedsections in the tensile test of the weld joints. That is, it was foundthat the classification of the forms of ruptured sections obtained bythe tensile test of weld joints corresponded with the classification ofthe fracture portion in the burst test of actual pipes. Based on theabove findings, the present inventors found that a burst at a pipe bodycould be attained by controlling the hardness of a base metal, thehardness of a HAZ and a peaking amount so as to satisfy the expression(3).

With respect to concrete control methods, a hardness can be controlledby controlling the chemical composition of a base metal itself andwater-cooling commencement and termination temperatures, a cooling rate,a welding heat input in TMCP and the like, and a peaking amount can becontrolled by controlling a curvature during C-pressing, a width duringU-pressing, an upsetting ratio during O-pressing and the like.

The reason why the range of the strength of a base metal is determinedto be 900 N/mm² or more is that, in case of a steel pipe of 800 N/mm²class, the degree of the softening of a HAZ against a base metal is notlarge enough, a strain concentrates on the HAZ during pipe expansion,and that leads easily to a rupture at the pipe body during a burst testeven though the steel pipe is hardened. In this connection, the presentinventors investigated the relation between a hardness and a tensilestrength and obtained the relation shown in FIG. 18.

Next, the present inventors studied concrete production indicators bywhich the expression (3) could easily be satisfied. In case of ahigh-strength steel pipe exceeding 900 N/mm² in tensile strength, as thesteel pipe is apt to generate cracking at seam weld during pipeexpansion, it is necessary to produce a steel pipe which does notgenerate pipe expansion cracking as a prerequisite of satisfying theexpression (3). A pipe forming test was carried out using the testsamples having the pipe expansion ratio of 0.8 to 1.2% and various wallthickness and outer diameters.

FIG. 17 shows the test samples which incurred pipe expansion crackingand the test samples which were successful in pipe expansion withoutincurring cracking at the welds in relation to the pipe body wallthickness. It was found that pipe expansion cracking could be preventedvery accurately if the relation between a peaking amount and a wallthickness satisfies the following expression (4),

 |δ|<40/t  (4).

The reason why a critical peaking amount is in inverse proportion to awall thickness is that the strain amount concentrating at each of thetoes of a weld tends to increase in proportion to the wall thickness.The reason why the number of tested samples is small on the negativepeaking side is that the test samples having negative peaking amountsgenerate the buckling of the bevels during O-pressing. The test samplescould be used for the test of this time by varying the curvatures in theaxial direction of the pipes during C-pressing or using a buckingprevention device during O-pressing.

EXAMPLES

The examples are explained hereunder.

Example 1

With regard to the invented examples and comparative examples in Example1, steel pipes were prepared by varying the specifications of the steelpipes, such as the steel sheet strength, the diameter after forming andthe wall thickness, and also varying the forming conditions of the steelpipes, such as the curvature in the range of 120 mm with a weld beforepipe expansion as the center R, the radius of a steel pipe after pipeexpansion at a specific expansion ratio r and the ratio R/r, as shown inTable 2. Then, with regard to the steel pipes thus prepared, the statusof a rupture at a seam weld during pipe expansion was observed, and,with regard to some of the steel pipes, the status of a rupture, theposition of a rupture and the form of a ruptured section in a hydraulicpressure burst test were also observed. The results of the observationsare shown also in Table 2. Further, the steel pipes 914.4 and 711.2 mmin outer diameter and 16, 12, 20 and 14 mm in wall thickness weresubjected to a hydraulic pressure burst test and the peaking values, theheights of the reinforcement of the weld metals at the inner surfaces,the rupture strength and the fracture portion were observed. The resultsof the observations are shown in Table 3.

As it was understood from Tables 2 and 3, though none of the steel pipesof the invented example Nos. 1 to 18 ruptured from the seam weld duringpipe expansion, some of the steel pipes ruptured from the seam welds orthe pipe bodies in the hydraulic pressure burst test and the rupturedsections were ductilely ruptured sections. On the other hand, any of thesteel pipes of the comparative example Nos. 1 to 5 ruptured from theseam weld during pipe expansion and could not be formed into a steelpipe. Further, though none of the steel pipes of the comparative exampleNos. 6 to 9 ruptured from the seam weld during pipe expansion, some ofthe steel pipes ruptured from the seam welds in the hydraulic pressureburst test and the ruptured sections were brittle ruptured sections.

TABLE 1 Conditions of analysis with finite element method Height ofreinforcement of weld metal Base Deposited Softened at inner Bevel metalmetal width of surface angle strength strength HAZ Case (mm) (degree)(MPa) (MPa) (mm) 1 1.4 40 940 1050 2 2 2.3 40 3 1.4 50

TABLE 2 Forming conditions Specification of steel pipe Curvature in theStrain Outer range of 120 mm Steel pipe at 4 mm Steel diameter with weldbefore Pipe radius point on Status sheet after Wall pipe expansionexpansion after pipe inner of seam strength forming thickness as thecenter R ratio expansion r surface during pipe Hydraulic pressure(N/mm²) (mm) (mm) (mm) (%) (mm) R/r (%) expansion burst test resultInvented example 1 1000 914 16 300 1.5 457 0.66 4.0 No rupture 2 860 91416 300 0.8 457 0.66 4.2 No rupture 3 1050 914 16 320 0.8 457 0.70 3.2 Norupture 4 1000 914 16 922 1.2 457 2.02 −3.5 No rupture 5 1020 914 11 3151.2 457 0.69 2.6 No rupture 6 1040 914 22 305 0.8 457 0.67 4.9 Norupture 7 930 609.4 16 234 1.2 304.7 0.77 3.8 No rupture 8 870 609.410.3 295 1.5 304.7 0.97 0.3 No rupture 9 870 1118 12 364 1.8 559 0.652.6 No rupture 10 950 1118 24 1105 1.8 559 1.98 −1.8 No rupture 11 1000914 16 425 0.8 457 0.93 2.6 No rupture Ruptured from seam, ductilelyruptured section 12 960 914 16 481 0.8 457 1.05 0.4 No rupture Rupturedfrom pipe body 13 960 914 16 922 0.8 457 2.02 −3.4 No rupture Rupturedfrom pipe body 14 870 609.4 10.3 295 1.5 304.7 0.97 2.5 No ruptureRuptured from pipe body 15 930 609.4 10.3 608 1.5 304.7 2.00 −1.7 Norupture Ruptured from pipe body 16 950 1118 24 1105 1.8 559 1.98 −2.2 Norupture Ruptured from pipe body 17 950 1118 24 512 1 559 0.92 2.9 Norupture Ruptured from seam, ductilely ruptured section 18 950 1118 24595 1 559 1.06 0.6 No rupture Ruptured from pipe body Comparativeexample 1 900 914 16 982 1 457 2.15 −5.5 Ruptured, failed in steel pipeforming 2 980 914 16 1089 1 457 2.38 −5.8 Ruptured, failed in steel pipeforming 3 980 914 11 252 1 457 0.55 4.5 Ruptured, failed in steel pipeforming 4 900 609.4 16 191 1 304.7 0.63 6.2 Ruptured, failed in steelpipe forming 5 900 1118 12 1353 1 559 2.42 −4.9 Ruptured, failed insteel pipe forming 6 1000 914 16 300 1.5 457 0.66 4.2 No ruptureRuptured, brittlely ruptured section 7 930 609.4 16 234 1.2 304.7 0.773.7 No rupture Ruptured, brittlely ruptured section 8 870 1118 12 3641.8 559 0.65 2.8 No rupture Ruptured, brittlely ruptured section 9 840914 16 252 1 457 0.55 6.8 No rupture Ruptured, brittlely rupturedsection

TABLE 3 Height of Peaking value (mm) reinforcement Steel Changed of weldmetal Outer Wall pipe Before amount after at inner diameter thicknessstrength pipe pipe surface Rupture Fracture (mm) (mm) (N/mm²) expansionexpansion (mm) strength portion Remarks 914.4 16 1010 0.3 1.2 1.7Identical to Burst at Invented pipe body seam example 955 0.6 0.5 1.4Identical to Burst at Invented pipe body pipe body example 1010 1.0 1.20.8 Identical to Burst at Invented pipe body seam example 955 −0.5 0.91.8 Identical to Burst at Invented pipe body pipe body example 955 −1.30.0 1.1 Identical to Burst at Invented pipe body pipe body example 10050.2 0.8 2.0 Identical to Burst at Invented pipe body pipe body example1010 0.8 0.6 1.8 Identical to Burst at Invented pipe body pipe bodyexample 860 0.7 1.4 1.2 Identical to Burst at Invented pipe body seamexample 1010 1.0 1.8 1.9 Identical to Burst at Invented pipe body seamexample 1005 0.9 1.0 1.7 Identical to Burst at Invented pipe body pipebody example 1005 0.0 0.0 1.0 Identical to Burst at Invented pipe bodypipe body example 1010 −1.5 −1.5 1.9 Identical to Burst at Invented pipebody pipe body example 1010 1.8 2.0 1.0 Lower than Burst at Comparativepipe body seam example 1005 1.5 0.3 2.2 Lower than Burst at Comparativepipe body seam example 955 1.3 1.5 1.6 Lower than Burst at Comparativepipe body seam example 955 1.9 2.1 1.4 Lower than Burst at Comparativepipe body seam example 1005 −2.3 −2.0 1.2 Lower than Burst atComparative pipe body seam example 1010 0.3 −1.0 2.4 Lower than Burst atComparative pipe body seam example 860 0.8 0.1 2.1 Lower than Burst atComparative pipe body seam example 914.4 12 1012 0.8 1.0 0.9 Identicalto Burst at Invented pipe body pipe body example 970 0.0 −0.5 1.4Identical to Burst at Invented pipe body pipe body example 970 1.3 1.21.2 Identical to Burst at Invented pipe body seam example 970 1.2 1.6 2Identical to Burst at Invented pipe body seam example 1012 2.5 2.1 1.5Lower than Burst at Comparative pipe body seam example 1012 1.5 1.8 2Lower than Burst at Comparative pipe body seam example 1012 0.2 −0.3 2.4Lower than Burst at Comparative pipe body seam example 914.4 20 940 0.81.2 1.9 Identical to Burst at Invented pipe body seam example 1000 −0.50.0 1.7 Identical to Burst at Invented pipe body pipe body example 10000.6 0.9 1.4 Identical to Burst at Invented pipe body pipe body example940 0.2 0.1 0.8 Identical to Burst at Invented pipe body pipe bodyexample 1000 2.3 2.0 1.4 Lower than Burst at Comparative pipe body seamexample 940 1.0 1.5 1.8 Lower than Burst at Comparative pipe body seamexample 940 0.5 −0.1 2.1 Lower than Burst at Comparative pipe body seamexample 711.2 14 980 0.6 1.3 1.4 Identical to Burst at Invented pipebody seam example 860 0.2 0.0 1.2 Identical to Burst at Invented pipebody pipe body example 1020 1.1 1.0 1.9 Identical to Burst at Inventedpipe body pipe body example 1050 2.8 2.0 1.7 Lower than Burst atComparative pipe body seam example 900 2.0 1.9 1.4 Lower than Burst atComparative pipe body seam example 1020 1.0 0.0 2.1 Lower than Burst atComparative pipe body seam example

Example 2

The effects of employing the present invention are shown in Tables 4 and5, shown hereunder, comparing the invented examples and the comparativeexamples. A rupture form index in Table 4 means the value obtained bysubtracting the left side value from the right side value of theexpression (3). As shown in Tables 4 and 5, in the expression (3)containing the hardness of a base metal, the hardness of a HAZ and apeaking amount or the expressions (3) and (4), when an index was minus,a brittlely ruptured section was observed in the tensile test and arupture occurred from a seam weld in the burst test. On the other hand,in the above expression (3) or expressions (3) and (4), when an indexwas plus as the case of an invented example, it was clear that a ruptureoccurred from a pipe body.

TABLE 4 Form of rupture Outer Wall Base metal HAZ Peaking Rupture Jointdiameter thickness hardness hardness value form tensile Burst (mm) (mm)Hv Hv (mm) index test test Remarks 914.4 16 321 265 1.8 −28 BrittleBurst at Comparative seam example 914.4 16 317 285 1.5 −34 Brittle Burstat Comparative seam example 914.4 16 320 275 1.5 −31 Brittle Burst atComparative seam example 914.4 16 312 284 0.9 −9 Brittle Burst atComparative seam example 914.4 16 313 270 0.3 15 Ductile Burst atInvented pipe body example 914.4 16 315 270 0.6 3 Ductile Burst atInvented pipe body example 914.4 16 304 278 1.0 19 Ductile Burst atInvented pipe body example 914.4 16 320 262 −0.5 1 Ductile Burst atInvented pipe body example 914.4 16 295 275 1.1 52 Ductile Burst atInvented pipe body example 914.4 12 315 285 1.8 −27 Brittle Burst atComparative seam example 914.4 12 321 275 1.0 −19 Brittle Burst atComparative seam example 914.4 12 321 278 1.8 −34 Brittle Burst atComparative seam example 914.4 12 315 265 1.0 5 Ductile Burst atInvented pipe body example 914.4 12 312 268 0.3 22 Ductile Burst atInvented pipe body example 914.4 12 321 260 0.5 3 Ductile Burst atInvented pipe body example 711.2 14 320 270 2.3 −35 Brittle Burst atComparative seam example 711.2 14 325 280 1.8 −47 Brittle Burst atComparative seam example 711.2 14 320 265 0.2 4 Ductile Burst atInvented pipe body example 711.2 14 308 265 1.6 11 Ductile Burst atInvented pipe bodv example 711.2 14 315 260 0.3 20 Ductile Burst atInvented pipe body example 711.2 14 305 265 2.0 13 Ductile Burst atInvented pipe body example

TABLE 5 Wall Outer thick- Peaking value (mm) diameter ness Range ofMeasured Pipe expansion (mm) (mm) invention value cracking Remarks 914.416 −2.5-2.5 3.2 Ruptured at weld Compar- 4.0 Ruptured at weld ative 2.7Ruptured at weld example −2.8 Ruptured at weld −3.0 Ruptured at weld 2.5No rupture Invented 1.8 No rupture example 1.5 No rupture 0.3 No rupture−0.5 No rupture −2.3 No rupture 914.4 12 −3.3-3.3 3.6 Ruptured at weldCompar- 3.8 Ruptured at weld ative example 1.8 No rupture Invented 1.0No rupture example 3.2 No rupture 0.3 No rupture 914.4 20 −2.0-2.0 2.5Ruptured at weld Compar- 2.3 Ruptured at weld ative example 2.0 Norupture Invented 1.6 No rupture example 1.0 No rupture 0.2 No rupture711.2 14 −2.8-2.8 3.2 Ruptured at weld Compar- 3.0 Ruptured at weldative example 2.8 No rupture Invented 2.2 No rupture example 1.5 Norupture 1.2 No rupture 0.8 No rupture 0.3 No rupture −1.2 No rupture−1.6 No rupture

What is claimed is:
 1. A high-strength steel pipe excellent informability, characterized in that, when a high-strength steel pipeexceeding 850 N/mm² in tensile strength is produced by a UOE method, aratio (R/r) of an average radius of curvature in a range of 120 mm in acircumferential direction including a weld of the steel pipe before pipeexpansion in a pipe expansion process (R) to a radius of the steel pipeafter pipe expansion (r) is 0.65 to 2.0.
 2. A high-strength steel pipeexcellent in formability and burst resistance, characterized in that,when a high-strength steel pipe exceeding 850 N/mm² in tensile strengthis produced by a UOE method, a ratio (R/r) of an average radius ofcurvature in a range of 120 mm in a circumferential direction includinga weld of the steel pipe before pipe expansion in a pipe expansionprocess (R) to a radius of the steel pipe after pipe expansion (r) is0.90 to 2.0.
 3. A high-strength steel pipe excellent in burstresistance, characterized in that, when a high-strength steel pipe 900N/mm² or more in tensile strength is produced by a UOE method, Vickershardness of a base metal of the steel pipe Hv, the Vickers hardness atHAZ Hz, pipe wall thickness t, and a peaking amount at a weld of thesteel pipe before pipe expansion in a pipe expansion process δ satisfyan expression (3), (1+0.005t|δ|)Hz<0.03584Hv ²−25.34Hv+4712  (3).
 4. Ahigh-strength steel pipe excellent in burst resistance according toclaim 3, characterized in that the peaking amount δ satisfies anexpression (4), |δ|<40/t  (4).